U.S. health inspectors found serious deficiencies at Theranos' northern California laboratory that, if not fixed, could see the lab suspended from the Medicare program, the Wall Street Journal reported, citing people familiar with the matter.

The problems were found during an inspection by the Centers for Medicare and Medicaid Services, the chief federal regulator of clinical labs, at Theranos' Newark facility, the WSJ reported.

The WSJ reported that it wasn't immediately clear what regulators faulted Theranos for in their latest inspection. The inspection results are expected to be publicly released soon, according to WSJ's sources.

Theranos spokeswoman Brooke Buchanan told the WSJ the company "does not have the report from last year's regularly scheduled CMS audit of its California lab."

She added that Theranos "has continued its ongoing work to build best-in-class systems and engage in partnership with its regulators."

As a blood-testing startup that claims to have developed hundreds of tests using a finger stick rather than the traditional venipuncture method, Theranos was initially welcomed as a healthcare disruptor. But an October expose by the WSJ ignited a series of accusations that Theranos had misled investors. A Fortune magazine story in December again raised scrutiny over the company.